Atomic clocks have been used in systems that require a very accurate time base or frequency measurement. Typical applications include global positioning systems (GPS) satellites, cellular phone systems, scientific experiments, and military applications. Conventional atomic clocks operate by use of an optical source (typically lamp-based, but some are laser-based) to “pump” atoms into the classical “0-0” state. The “0-0” state is the transition between an upper energy level with azimuthal quantum number 0 and total angular momentum quantum number f=I+½ and a lower energy level with azimuthal quantum number 0 and total angular momentum quantum number f=I−½. In such systems, a microwave field is coupled into a microwave cavity enclosing an atomic vapor cell and, operating under feedback control, the microwave frequency of the microwave field is locked to the atomic 0-0 frequency state. The locking on to the atomic 0-0 frequency state by means of an applied microwave frequency field is called RF-interrogation.
In many existing and emerging applications, it is desirable that the dimensions of the atomic clock be small with low power consumption requirements. If the dimensions of the atomic clock are sufficiently small, then many parts comprising the clock can be manufactured using similar batch fabrication techniques as those found in the semiconductor industry.
Limitations of conventional atomic clocks include the use of microwave cavities which limit the clock dimensions as well as place a limit on performance/applications due to cavity pulling effects. A microwave cavity was needed in order to produce a uniform RF field of sufficient RF power when using larger bulb-style alkali-vapor cells.
As is known in the art, the microwave cavity can be eliminated by use of a modulated optical source in a technique called Coherent Population Trapping: (CPT). By modulating the optical source at the atomic hyperfine frequency (the 0-0 frequency), the optical sidebands can interact with the atoms in a way analogous to direct RF interrogation, yet without needing bulky microwave cavity components. In CPT, an optical laser is modulated with microwave power. The laser responds by generating optical sidebands that are positioned about that main laser line at a frequency equal to the modulating RF frequency (or at a harmonic or sub-harmonic). Feedback electronics is used to lock the frequency of an electronic VCO to the atomic hyperfine resonance, which is sensed by the interaction of the coherent optical frequency sideband spectrum and the alkali atoms.
A novel variant is to utilize the atomic “end-transition” as opposed to the classical 0-0 transition. The end-transition allows for scaling the clock physics package (reduced dimensions) without seriously affecting clock performance. End transition interrogation allows for optical pumping of atoms into a common state (maximum or minimum of angular momentum), thereby providing for increased signal strength, suppression of spin-exchange broadening, and with high buffer gas pressure, allowing for scaling of alkali-vapor cells to the millimeter or sub-millimeter dimensions. End-transition interrogation allows for the production of strong signals (high performance) as cell size is reduced, which in turn allows for the production of an extremely compact, low power dissipation, yet high performance atomic clock source.
Several end-transition architectures can be implemented, including ones that use a microwave cavity or are CPT-based. For millimeter-scale alkali-vapor cells, direct RF-interrogation is the preferred approach: no microwave cavity is needed as the vapor cells (and feed loops) are of dimensions less than one RF wavelength (9.19 GHz for cesium; i.e. a wavelength of about 3 cm in free space). Therefore, RF uniformity and drive power are satisfied without need for microwave resonator. Further, incomplete polarization pumping from a single circularly polarized laser reduces the effectiveness of end-transition CPT techniques.
As the end-transitions are linearly dependent on local magnetic fields, an approach is taken to actively lock the local field to a pre-determined value. This approach involves the direct sensing of an atomic resonance used as a measure of the local field value, and with feedback electronics, maintains the local bias field at a constant value.
Long-term stability of optically-pumped clocks is degraded due to the varying amount of optical power being absorbed by the atoms, so-called “light shifts”. An advantage of the end-state approach is that, since light shifts (AC Stark shifts) look like magnetic field shifts, an active magnetic field feedback system can be designed to also actively compensate for light shifts, a technique not possible with designs based on the 0-0 transition, which is quadratically dependent on magnetic field.
Accordingly, what would be desirable, but has not yet been provided, is high performance, compact (and scalable), low power atomic time/frequency reference that employs direct RF-interrogation on the end-state transition. Such a system can include compact, non-contact RF-interrogation as well as appropriate bias-field input and control.